Split configuration file into multiple files

We have three ways to split a large configuration file into multiple small configuration files. Here is the introductions.

1. <linkedConfiguration>
   The main limitation of this way is the <linkedConfiguration> element is not supported for applications with Windows side-by-side manifests. We can refer to MSDN to get more information.
   
2. System.Configuration.ConfigurationManager or System.Web.Configuration.WebConfigurationManager
   The main defect is we must design an architecture by ourself for supporting this. We can refer to MSDN to get more information.
   
3. Using configSource
   Base on my understanding, this is the best way, we can avoid the previous two defects by the way. Here is my example. I use Unity Application Block of Microsoft Enterpriese Library in the example; I separated unity configuration file into another file under a sub folder.
   
   Code
   namespace Learning.WebApplication
   {
       public partial class _Default : System.Web.UI.Page
       {
           protected void Page_Load(object sender, EventArgs e)
           {
               IUnityContainer unityContainer = new UnityContainer();
               UnityConfigurationSection section = (UnityConfigurationSection)ConfigurationManager.GetSection("unity");
               section.Containers.Default.Configure(unityContainer);
   
               Test obj = unityContainer.Resolve<Test>();
               if (obj == null)
                   Response.Write("null");
               else
                   Response.Write(obj.GetType().ToString());
           }
       }
   
       public class Test { }
   }
   
   
   Web.config (Partial)
   <?xml version="1.0"?>
   <configuration>
     <configSections>
       <section name="unity" type="Microsoft.Practices.Unity.Configuration.UnityConfigurationSection, Microsoft.Practices.Unity.Configuration, Version=1.2.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
     </configSections>
     <unity configSource="App_Data\Unity.config" />
   </configuration>
   
   
   Unity.config
   <?xml version="1.0" ?>
   <unity>
     <typeAliases>
       <typeAlias alias="Test" type="Learning.WebApplication.Test, Learning.WebApplication" />
     </typeAliases>
     <containers>
       <container>
         <types>
           <type type="Test" />
         </types>
       </container>
     </containers>
   </unity>